Kinetic Studies on Polymerization of Poly (LACTIC ACID) Using Suitable Catalyst

Abstract

Biodegradable polymer materials are of great interest because these are used in packaging, agriculture, medicine and other areas. Poly (lactic acid), PLA is one of the most promising biodegradable polymers (biopolymers) and has been the subject of abundant study over the last decade. This thesis embodies the subject matter resulting out of this study. The entire work is arranged in five chapters. A brief introduction, historical background and the applications of biodegradable polymers (plastics) are described first. The importance of lactic acid, monomer used in the synthesis of polylactide, is also discussed. A brief account of physical characteristics and advantages and disadvantages of PLA is presented. Ring-opening polymerization technique is more useful for the production of high molecular weight PLA than polycondensation, which yield low molecular weight of PLA. For an estimation of kinetic parameters, mathematical modeling of the ring-opening polymerization is also described. Both the methods for the synthesis of PLA from lactide are described and out of these the ring-opening polymerization method has been chosen for the present work because it gives higher molecular weight of PLA. Three major reaction mechanisms: cationic, anionic, and coordination-insertion are discussed. However, high molecular weight polyesters have only been obtained by using anionic or coordination-insertion ring-opening polymerization. Studies on the effects of different parameters: polymerization temperature, polymerization time, monomer/initiator ratio, nature of the initiator, amount of water or other impurities etc. which affect the polymerization of lactide are reviewed. Determination of kinetic rate constants by modeling and simulation and polymerization mechanism is also discussed. Relevant literature for the determination of intrinsic viscosity by using Ubbelohde viscometer (Mark-Houwink equation) is also reviewed. Ring-opening polymerization has been studied wherein the starting monomer is L- lactide. The importance of recrystallization of monomer in synthesis of PLA is also explained. To carry out ring-opening polymerization of lactide, the experimental set-up was housed in a fume hood. Some polymerization reactions were carried out under dry nitrogen atmosphere and some reactions were carried out under vacuum only. Synthesis of polylactide under inert atmosphere and vacuum are briefly explained. Viscometry method, for the determination of intrinsic viscosity of synthesized PLA samples, is described. Synthesis of PLA has been carried out under two different environments, an inert atmosphere and vacuum. Mark-Houwink parameters have been determined for intrinsic viscosity and average molecular weight. Synthesis of polylactide was carried out by using various initiators like stannous octoate, dibutyltindimethoxide, zinc stearate and one co-initiator, triphenylphosphine. These initiators were chosen because all of them have bulky groups attached to the metal atom which would provide the steric hindrance around metal atom during polymerization reaction. There would be possibility of the large polymer chains to be produced during reaction due to the bulky groups attached to metal atom. Zinc stearate is used in a large scale in chemical industries. Dispersion of initiator in monomer with a solvent (diethylether) was found to be a very important factor during polymerization. If the dispersion of initiator is not proper, one may get pockets of less as well as more number of initiator molecules (and also more number of growing polymer chains). The poor dispersion has been related with the bimodality or multimodality observed in the SEC chromatogram of the product so formed. With proper dispersion, bimodality or, multimodality in the SEC chromatogram of the product was reduced to single or unimodal peak. It has been reported in the literature that triphenylphosphine (as co-initiator) helps in increasing the molar mass of polylactide. The molecular weight of polylactide obtained in the present study using stannous octoate was upto thousands but when triphenylphophine was used as co-initiator, molecular weight increased from thousand to lakhs. High molecular weight PLA has been obtained only in case of stannous octoate and stannous octoate/triphenylphosphine. While in the case of dibutyltindimethoxide and dibutyltindimethoxide/triphenylphosphine, low molecular weight of polylactide up to a few thousands has been obtained because these initiators are known to be effective transesterification catalysts and also known to cause ‘back-biting’ degradation. Zinc stearate (initiator) gave lower molecular weight PLA than stannous octoate but higher than dibutyltindimethoxide because it is a weaker base, require higher nucleophilicity to initiate lactide and that too only at higher temperatures. Some experiments were also performed with zinc stearate at 180 °C, but degradation took place at this temperature. Anionic ring-opening polymerization mechanisms of synthesized polylactide with various initiators have been proposed. A simple and reliable model has been presented for the polymerization of lactide to PLA. The model enables numerical solution of rate equations for initiation, propagation, and termination steps. It is easily extendable to more complex polymerization mechanisms. The simulation can be done in conjunction with the experimental data to yield individual rate constants. It is possible to obtain unique values for various rate constants using Mn versus time and polydispersity data. Accurate rate constants can be predicted using appropriate and reproducible rate data. This methodology offers greater opportunity for capturing high, non-equilibrium polymer yield through appropriately timed termination of the polymerization reaction. A comparison of polymerization kinetics (polymerization carried out under two different environments: nitrogen atmosphere, vacuum) has been done when the initiator used is stannous octoate with and without triphenylphosphine. It is interesting that the propagation rate constant, kp, is same for both the initiators. This means that the polymer chain once initiated will grow at the same rate in both cases (also, there is an assumption in the analysis that the propagation rate constant is independent of the chain length). The initiation rate constant, ko, is comparatively very less for pure stannous octoate initiator (nitrogen atmosphere). This would also explain the high experimental values of PD (implying a very broad molecular weight distribution). The growing polymer chains will start and terminate at different time leading to a broad MWD. In contrast, for stannous octoate with triphenylphosphine initiator, the experimental PD is very less which would be a desirable attribute of this system. For the vacuum atmosphere polymerizations, ko, is identical for both stannous octoate and stannous octoate/triphenylphosphine. Thus, it is possible that the nitrogen atmosphere provided in the reaction kettle somehow decrease the rate constant. The termination rate constant, kt, for pure stannous octoate initiator is about 2.6 times that for stannous octoate with triphenylphosphine initiator. This could be the major reason for the difference in the average molecule weights achieved in PLA synthesis in the two cases. In case of vacuum, kt, is same for both.